Broad spectrum ultraviolet catadioptric imaging system

ABSTRACT

An ultraviolet (UV) catadioptric imaging system, with broad spectrum correction of primary and residual, longitudinal and lateral, chromatic aberrations for wavelengths extending into the deep UV (as short as about 0.16 μm), comprises a focusing lens group with multiple lens elements that provide high levels of correction of both image aberrations and chromatic variation of aberrations over a selected wavelength band, a field lens group formed from lens elements with at least two different refractive materials, such as silica and a fluoride glass, and a catadioptric group including a concave reflective surface providing most of the focusing power of the system and a thick lens providing primary color correction in combination with the focusing lens group. The field lens group is located near the intermediate image provided by the focusing lens group and functions to correct the residual chromatic aberrations. The system is characterized by a high numerical aperture (typ. greater than 0.7) and a large flat field (with a size on the order of 0.5 mm). The broad band color correction allows a wide range of possible UV imaging applications at multiple wavelengths.

TECHNICAL FIELD

The present invention relates to optical systems adapted for imaging inthe ultraviolet (UV) portion of the spectrum, and in particular tobroadband UV catadioptric imaging optics, i.e. systems employing acombination of one or more lens elements and one or more reflecting(mirror) elements in series. The invention is addressed especially tosystems that have been designed to correct for imaging and coloraberrations.

BACKGROUND ART

Catadioptric imaging systems for the deep ultraviolet spectral region(about 0.15 to 0.30 μm wavelength) are known. U.S. Pat. No. 5,031,976 toShafer and U.S. Pat. No. 5,488,229 to Elliott and Shafer disclose twosuch systems. These systems employ lens elements made from only a singlerefractive material, namely fused silica, since it is practically theonly material that combines good transmission of deep UV light withdesirable physical properties. For example, fluoride glasses (based onCaF₂, LiF, etc.), while transmissive of deep UV light, are generallyconsidered too soft, making lens formation difficult. Thus, fluorideglass materials are normally avoided whenever possible.

In the above-noted '976 Shafer patent, an optical system is disclosed,which is based on the Schupmann achromatic lens principle producing anachromatic virtual image, and which combines it with a reflective relayto produce a real image. The system, reproduced here as FIG. 7, includesan aberration corrector group of lenses 101 for providing correction ofimage aberrations and chromatic variation of image aberrations, afocusing lens 103 receiving light from the group 101 for producing anintermediate image 105, a field lens 107 of the same material as theother lenses placed at the intermediate image 105, a thick lens 109 witha plane mirror back coating 111 whose power and position is selected tocorrect the primary longitudinal color of the system in conjunction withthe focusing lens 103, and a spherical mirror 113 located between theintermediate image and the thick lens 109 for producing a final image115. Most of the focusing power of the system is due to the sphericalmirror 113. It has a small central hole near the intermediate image 105to allow light from the intermediate image 105 to pass therethrough tothe thick lens 109. The mirror coating 111 on the back of the thick lens109 also has a small central hole 119 to allow light focused by thespherical mirror 113 to pass through to the final image 115. Whileprimary longitudinal (axial) color is corrected by the thick lens 109,the Offner-type field lens 107 placed at the intermediate image 105 hasa positive power to correct secondary longitudinal color. Placing thefield lens slightly to one side of the intermediate image 105 correctstertiary longitudinal color. Thus, axial chromatic aberrations arecompletely corrected over a broad spectral range. The system incidentlyalso corrects for narrow band lateral color, but fails to providecomplete correction of residual (secondary and higher order) lateralcolor over a broad UV spectrum.

The above-noted '229 patent to Elliott and Shafer provides a modifiedversion of the optical system of the '976 patent, which has beenoptimized for use in 0.193 μm wavelength high power excimer laserapplications, such as ablation of a surface 121', as seen in FIG. 8.This system has the aberration corrector group 101', focusing lens 103',intermediate focus 105', field lens 107', thick lens 109', mirrorsurfaces 111' and 113' with small central openings 117' and 119' thereinand a final focus 115' of the prior '976 patent, but here the field lens107' has been repositioned so that the intermediate image or focus 105'lies outside of the field lens 107' to avoid thermal damage from thehigh power densities produced by focusing the excimer laser light.Further, both mirror surfaces 111' and 113' are formed on lens elements108' and 109'. The combination of all light passes through both lenselements 108' and 109' provides the same primary longitudinal colorcorrection of the single thick lens 109 in FIG. 7, but with a reductionin total glass thickness. Since even fused silica begins to haveabsorption problems at the very short 0.193 μm wavelength, the thicknessreduction is advantageous at this wavelength for high power levels.Though the excimer laser source used for this optical system has arelatively narrow spectral line width, the dispersion of silica near the0.193 μm wavelength is great enough that some color correction stillneeds to be provided. Both prior systems have a numerical aperture ofabout 0.6.

Longitudinal chromatic aberration (axial color) is an axial shift in thefocus position with wavelength. The prior system seen in FIG. 7completely corrects for primary, secondary and tertiary axial color overa broad wavelength band in the near and deep ultraviolet (0.2 μm to 0.4μm). Lateral color is a change in magnification or image size withwavelength, and is not related to axial color. The prior system of FIG.7 completely corrects for primary lateral color, but not for residuallateral color. This is the limiting aberration in the system when abroad spectral range is covered.

An object of the invention is to provide a catadioptric imaging systemwith correction of image aberrations, chromatic variation of imageaberrations, longitudinal (axial) color and lateral color, includingresidual (secondary and higher order) lateral color correction over abroad spectral range in the near and deep ultraviolet spectral band (0.2to 0.4 μm).

In addition to color correction, it is also desired to provide a UVimaging system useful as a microscope objective or as microlithographyoptics with a large numerical aperture for the final image and with afield of view of at least 0.5 mm. The system is preferably telecentric.

DISCLOSURE OF THE INVENTION

The object is met with a catadioptric imaging system in which anachromatic multi-element field lens is used, made from two or moredifferent refractive materials, such as fused silica and fluoride glass.The field lens may be a doublet or preferably a triplet, which may becemented together or alternatively spaced slightly apart. Because fusedsilica and fluoride glass do not differ substantially in dispersion inthe deep ultraviolet, the individual powers of the several componentelements of the field lens need to be of high magnitude. Use of such anachromatic field lens allows not only axial color, but also lateralcolor to be completely corrected over a broad spectral range. Only onefield lens component need be of a different refractive material than theother lenses of the system.

An optical system according to the present invention includes a focusinglens group with plural lens elements, preferably all formed from asingle type of material, with refractive surfaces having curvatures andpositions selected to focus light to an intermediate image with highlevels of correction in the final image of both image aberrations andchromatic variation of aberrations over a UV wavelength band of at least0.20 to 0.29 μm, and preferably extending over 0.20 to 0.40 μm. Systemsadapted for a UV band that includes the 0.193 μm wavelength are alsopossible. The system also includes the aforementioned field lens grouppositioned near the intermediate image to provide correction ofchromatic aberrations including residual axial and lateral color. Theintermediate image plane may be located either inside or outside thefield lens group depending on the optimization. A catadioptric groupincludes a concave spherical reflector, which may either be a mirror ora reflectively coated lens element, and a planar or near planarreflector near the final image, which is a reflectively coated lenselement. Both reflective elements have central optical apertures thereinwhere reflective material is absent, allowing light from theintermediate image to pass through the concave reflector, be reflectedby the planar (or near planar) reflector onto the concave reflector, andpass through the planar (or near planar) reflector, traversing theassociated lens element or elements on the way.

The imaging system provides a numerical aperture of at least 0.7, alarge field size of about 0.5 μm and substantially flat field imagingover a broad wavelength band extending into the deep UV portion of thespectrum. The system is useful in a number of optical configurations,including bright field illumination, directional and ring(nondirectional) dark field illumination, fluorescence imaging, full skyscatterometer and confocal microscope configurations. UV imaging systemsprovide not only better optical resolution, but also better materialidentification due to strong variations in the reflectivity andabsorption of UV light by materials, strong scattering (proportional toλ⁻⁴), higher orders of diffraction, and fluorescence in the UV spectrum.Broad band UV imaging systems can have UV lamps as light sources, whichprovide incoherent light for no speckle imaging, and which enable otherspecial imaging techniques, such as Nipkow disk type confocalmicroscopy, to be used in the UV spectrum. Possible applications for thebroad band, deep UV objective lens include wafer and photomaskinspection, material masking and cutting applications, UV lithography,biological microscopy, metallurgical microscopy, spectroscopic analysisof specimen materials, and others.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a catadioptric imaging system inaccord with the present invention.

FIG. 2 is an enlarged portion of the imaging system of FIG. 1 in thevicinity of the intermediate image 13 showing elements of an achromaticfield lens group for the system.

FIG. 3 is an enlarged portion, comparable to FIG. 2, of a catadioptricimaging system in accord with the present invention showing elements ofan alternative achromatic field lens group for the system.

FIG. 4 is a schematic side view of a catadioptric tube lens designed toaccompany the imaging system of FIG. 1 when used as aninfinity-corrected microscope objective.

FIG. 5 is a schematic side view of a portion of a catadioptric imagingsystem in accord with the present invention used for a dark-field lightscatter imaging wafer inspection device, showing an oblique laser beamillumination source.

FIG. 6 is a schematic side view of a wafer inspection apparatusemploying the catadioptric imaging system of the present invention as aUV objective of the inspection apparatus.

FIGS. 7 and 8 are schematic side views of catadioptric imaging systemsof the prior art.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIG. 1, a catadioptric imaging system of the presentinvention, which is especially suited for use in broad-banddeep-ultraviolet applications, is made up of a focusing lens group 11forming an intermediate image 13, a field lens group 15 disposedproximate to the intermediate image 13 for providing correction ofchromatic aberrations, and a catadioptric group 17 focusing the lightfrom the intermediate image 13 to a final image 19. The imaging systemis optimized to correct both monochromatic (Seidel) aberrations andchromatic aberrations (longitudinal and lateral), as well as chromaticvariations of the monochromatic aberrations, over a wavelength band thatextends into the deep ultraviolet (UV) portion of the spectrum,including at least 0.20 to 0.29 μm UV light and preferably extendingover a broad band covering 0.20 to 0.40 μm UV light. Both ranges includethe 0.248 μm KrF excimer laser line and the 0.248 μm and 0.254 μmmercury emission lines. The broader spectral range also includes the0.365 μm mercury emission line (commonly known as the i-line), the 0.351μm XeF excimer laser line, and the 0.325 μm He-Cd laser line. A wideassortment of other laser and arc lamp emission wavelengths in theultraviolet are also available. The system could also be adapted toprovide chromatic-aberration-corrected imaging over other UV wavelengthranges. For example, a 0.19 to 0.40 μm wavelength band that includes the0.193 μm ArF excimer laser line is also possible. Narrower bands mightalso be used. The catadioptric system of the present invention can beadapted for a number of UV imaging applications, including as a UVmicroscope objective, a collector of surface scattered UV light in awafer inspection apparatus, or as mask projection optics for a UVphotolithography system.

The focusing lens group 11 in FIG. 1 consists of seven lens elements21-27, with two of the lenses 21 and 22 separated by a substantialdistance from the remaining five lens elements 23-27. In particular, theseparation of the pair of lenses 21 and 22 from the remaining five lenselements 23-27 in this focusing lens group is typically on the order ofat least one-half the total combined thickness of the five lens elements23-27 forming the main focusing subgroup. For example, lens elements23-27 may span a distance of about 60 mm and lens element 22 may be 30to 60 mm from lens element 23. The actual dimensions depend on the scalechosen for the design. The two lenses 21 and 22 form a nearly zero-powerdoublet for the correction of chromatic variation of monochromatic imageaberrations, such as chromatic variation of coma and astigmatism. Byhaving this doublet 21 and 22 relatively far from the rest of the systemcomponents, the shift of the light beam on these two lenses with fieldangle is maximized. That, in turn, helps greatly in achieving the bestcorrection of chromatic variation of aberrations. The five lenses 23-27of the main focusing subgroup in FIG. 1 consist of a thick strongnegative meniscus lens 23, an opposite-facing strongly-curved negativemeniscus lens 24, a strong biconvex lens 25, a strong positive meniscuslens 26, and an opposite-facing strongly-curved, but very weak, meniscuslens 27 of either positive or negative power. Variations of thissubgroup of lens 23-27 are possible. The subgroup focuses the light toan intermediate image 13. The curvature and positions of the lenssurfaces are selected to minimize monochromatic aberrations and also tocooperate with the doublet 21-22 to minimize chromatic variations ofthose aberrations.

The field lens group 15, seen in an expanded view in FIG. 2, typicallycomprises an achromatic triplet, although a doublet might also be used.Both fused silica and CaF₂ glass materials are used. Other possible deepUV transparent refractive materials can include MgF₂, SrF₂, LaF₃ and LiFglasses, or mixtures thereof. Note, however, that some of thesematerials can be birefringent if they are not completely amorphous andcontain microcrystals. Because the dispersions between the two UVtransmitting materials, CaF₂ glass and fused silica, are not verydifferent in the deep ultraviolet, the individual components of thegroup 15 are quite strong. The triplet 15 may comprise a fused silicanegative meniscus lens 31, a CaF₂ biconvex (positive) lens 33 and afused silica biconcave (negative) lens 35, all cemented together. Theoptimized design for this configuration places the intermediate image 13inside the triplet group 15. Alternatively, as seen in FIG. 3, theachromatic field lens group may comprise two fused silica, oppositefacing negative meniscus lenses 51 and 53, spaced slightly apart (typ.about 1.0 mm), followed by a CaF₂ biconvex (positive) lens 55 nearlyabutting the second of the meniscus lenses 53. The optimized design forthis second configuration allows the intermediate image 13 to be formedoutside the field lens group 15 beyond the CaF₂ lens. Either embodimentof the field lens group 15 has surface curvatures and positions selectedto correct residual (secondary and tertiary) axial and lateral color.Primary color aberrations are corrected mainly by the lens elements inthe catadioptric group 17 in combination with the focusing lens group11. Use of two or more different refractive material types in the fieldlens groups, such as both fused silica and CaF₂ glass, allows residuallateral color to be completely corrected, in addition to the axial colorcorrections provided by prior single-material field lenses.

As seen in FIGS. 2 and 3, the intermediate focus 13 may be locatedeither inside or outside of the field lens group 15. If the intermediateimage 13 is inside the group 15, maximum aberration correction isachieved. Alternatively, it may be desirable to have the image 13outside the field lens group 15 in cases where there is danger that highoptical power densities may cause damage to the glass material of one ormore of the field lens elements. Furthermore, small imaging errors dueto glass inhomogeneities are less of a factor when the field lens group15 is placed somewhat away from the intermediate image 13.

The catadioptric group 17 seen in FIG. 1 includes a first opticalelement consisting of a fused silica meniscus lens 39 with a concavereflective surface coating 41 on a back surface of the lens 39, and alsoincludes a second optical element consisting of a first silica lens 43with a reflective surface coating 45 on a back surface of the lens 43.(The front surfaces of the two lens elements 39 and 43 of thecatadioptric group 17 face each other.) The reflective surface coatings41 and 45 are typically composed of aluminum, possibly with a MgF₂overcoat to prevent oxidation. Aluminum has a nearly uniformreflectivity of at least 92% over the entire near and deep UV wavelengthrange. Other metals commonly used as reflective coatings in the visibleportion of the spectrum have reflectivities that vary considerably withwavelength or even become opaque in the deep UV. For example, silverdrops to only 4% reflectivity at 0.32 μm. Possible alternatives toaluminum, but with somewhat lower reflectivities near 60%, includemolybdenum, tungsten and chromium. These may be favored in certain highpower applications, such as laser ablation. Specialized coatings,including long-wave pass, short-wave pass and band pass dichroicreflective materials, partially transmissive and reflective materialcoatings, and fluorescent coatings, could all be used for a variety ofspecialized applications.

The first lens 39 has a hole 37 centrally formed therein along theoptical axis of the system. The reflective coating 41 on the lenssurface likewise ends at the central hole 37 leaving a central opticalaperture through which light can pass unobstructed by either the lens 39or its reflective coating 41. The optical aperture defined by the hole37 is in the vicinity of the intermediate image 13 so that there isminimum optical loss. The achromatic field lens group 15 is positionedin or near the hole 37. The second lens 43 does not normally have ahole, but there is a centrally located opening or window 47 on thesurface reflective coating 45 where the coating is absent, leavinganother optical aperture at the central window location 47. The opticalaperture in lens 39 with its reflective coating 41 need not be definedby a hole 37 in the lens 39, but rather could simply be defined by awindow in the coating 41 where reflective coating material is absent,just as with lens 43 and coating 45. In that case, light would pass oneadditional time through the refractive surfaces of lens 39.

The coated mirror 45 may be either flat or preferably slightly curved.The slight curvature will provide some centering tolerance for thatelement. Moreover, if the reflective element 45 is slightly curved, itwill make contact with a wafer surface or other object to be imaged bythe catadioptric system less likely and avoid the damage to both mirrorcoating 45 and the object which would result from any such contact.

Light from the intermediate image 13 passes through the optical aperture37 in the first lens 39 then through the body of the second lens 43where it is reflected back through the body of the second lens 43 by theplanar or near planar mirror coating 45 on the back surface of the lens43. The light then passes through the first lens 39, is reflected by themirror surface 41 and passes back through the body of the first lens 39.Finally the light, now strongly convergent passes through the body ofthe second lens 43 for a third time, through the optical aperture 47 tothe final image 47. The curvatures and positions of the first and secondlens surfaces are selected to correct primary axial and lateral color inconjunction with the focal lens group 11.

The optical elements may be assembled with or without cementedinterfaces. Cementing lens elements simplifies assembly, resulting in aless expensive objective. It also results in a more robust device inwhich the cemented elements are far less likely to go out of alignment.Moreover, the cementing process can be used to seal environmentalsensitive materials, such as the CaF₂ field lens element between otherelements. On the other hand, since the polymeric materials used ascement in lens systems can be damaged by deep UV light possibly leadingto degradation of the optical system and providing uncertain lifetime insome high power UV applications, non-cemented systems will be preferredin those high power deep UV system applications where long-termreliability is a significant issue. This is an important considerationin the selection of a cemented or non-cemented design for the field lensgroup elements located near the intermediate image where UV radiation ismost concentrated.

Specific values for two examples of optimized broad-band system designs,one for the field lens group of FIG. 2 and the other for the alternatefield lens group of FIG. 3, follow. The lens surface data are based onrefractive index values (relative to air) for the wavelengths 0.200,0.220, 0.245, 0.290 and 0.400 μm. The resulting designs have a numericalaperture of about 0.9 and a field size of about 0.5 mm diameter.Variations of the design can be done for a slightly lower numericalaperture, for example about 0.7, by simply reoptimizing the surfacecurvatures for the desired parameters. Such a variation would besuitable for reticle inspection, where longer working distances arepreferred. Likewise, with slight adjustments to the surface curvatures,and allowing for a narrower wavelength band over which longitudinal andlateral are corrected, the system can be optimized to include 0.193 μmArF excimer laser light over a wide band of 0.19 to 0.40 μm or over anarrower band.

    ______________________________________                                                 Radius of                                                            Surface #                                                                              Curvature (mm)                                                                            Spacing (mm) Material                                    ______________________________________                                        Lens Data (Embodiment #1)                                                     1        1105.7      4.000        fused silica                                2        53.051      2.500        air                                         3        284.061     5.000        fused silica                                4        -57.181     60.000       air                                         5        39.782      15.000       fused silica                                6        13.379      7.977        air                                         7        -12.955     5.169        fused silica                                8        -17.192     1.000        air                                         9        42.964      8.000        fused silica                                10       -55.269     1.000        air                                         11       18.436      8.000        fused silica                                12       91.574      6.253        air                                         13       -20.802     4.000        fused silica                                14       -21.768     17.120       air                                         15       7.217       5.494        fused silica                                16       2.259       3.000        CaF.sub.2 glass                             17       -11.760     1.500        fused silica                                18       373.721     39.730       air                                         19       flat        7.000        fused silica                                20       flat        -7.000       reflector/                                                                    fused silica                                21       flat        -36.000      air                                         22       50.470      -9.500       fused silica                                23       64.290      9.500        reflector/                                                                    fused silica                                24       50.470      36.000       air                                         25       flat        7.000        fused silica                                26       flat        1.500        air                                         Lens Data (Embodiment #2)                                                     1        -67.007     4.000        fused silica                                2        50.308      2.000        air                                         3        120.297     6.000        fused silica                                4        -37.494     30.636       air                                         5        24.138      10.000       fused silica                                6        13.441      9.532        air                                         7        -13.518     7.546        fused silica                                8        -17.997     1.000        air                                         9        34.465      6.000        fused silica                                10       -517.022    1.000        air                                         11       18.268      10.000       fused silica                                12       965.352     4.181        air                                         13       -30.177     9.746        fused silica                                14       -28.138     7.892        air                                         15       -19.346     2.500        fused silica                                16       -36.530     1.000        air                                         17       6.687       5.026        fused silica                                18       2.044       0.017        air                                         19       2.044       2.000        CaF.sub.2 glass                             20       -90.635     36.108       air                                         21       -908.968    7.000        fused silica                                22       -1000.0     -7.000       reflector/                                                                    fused silica                                23       -908.968    -36.000      air                                         24       48.244      -9.500       fused silica                                25       63.204      9.500        reflector/                                                                    fused silica                                26       48.244      36.000       air                                         27       -908.968    7.000        fused silica                                28       -1000.0     1.500        air                                         ______________________________________                                    

With reference to FIG. 4, a tube design for using the imaging system ofFIG. 1 as a microscope objective is shown. Illumination of a samplesurface being imaged by the objective of FIG. 1 may be made through theobjective itself, by means of an ultraviolet light source 61, such as amercury vapor lamp or excimer laser, together with conventionalillumination optics 63, 65, 67, leading to a beamsplitter 69 in theobjective's optical path. The imaging path for light received from theobjective of FIG. 1 is via transmission through the beamsplitter 69 to amicroscope tube, whose design may also be catadioptric. The tubeelements include a pair of opposite facing negative meniscus lenses 71and 73 closely spaced to one another, and two spherical mirrors 75 and77 spaced from each other and from the pair of lenses 71 and 73 by atleast 400 mm. The curvature of mirror 75 is concave toward the lenses 71and 73 and the mirror 77, while the curvature of mirror 77 is convextoward the mirror 75, both curvatures being at least 1000 mm radius,i.e. nearly flat. The mirrors 73 and 75 fold the optical path off-axisso that the system length is under 500 mm. One example optimized for theparticular objective seen in FIG. 1 has the following characteristicrefractive and reflective surfaces for optical elements 71, 73, 75 and77:

    ______________________________________                                        Surface  Radius of                                                            #        Curvature (mm)                                                                            Spacing (mm) Material                                    ______________________________________                                        1        -92.965     4.989        fused silica                                2        -96.420     1.000        air                                         3        89.440      4.616        fused silica                                4        87.394      413.186      air                                         5        -1219.4     -413.186     reflector                                   6        -1069.7     463.186      reflector                                   ______________________________________                                    

Referring now to FIG. 5, yet another use for the imaging system of FIG.1 is for wafer inspection, namely as a directional dark field, scatteredlight collector. A UV laser illumination source 81 directs a beam 85through holes 83 and 87 formed in lenses 39" and 43" and reflectivecoatings 41" and 45" of the catadioptric group onto a surface 89 to beinspected. Alternatively, only the reflective coatings 41" and 45" mightbe absent or only partially reflective to form transparent or at leastpartially transmissive windows for the light beam 85. The beam 85 mightalso enter the system from below the hemispherical reflector 41". Theangle of incidence is oblique, i.e. at least 60° from vertical due tothe high numerical aperture (about 0.90) of the imaging system.Illumination may be done from more than one direction and angle ofincidence. The specularly reflected light 93 passes through holes 91 and95 formed in lenses 39" and 41" and reflective coatings 41" and 45" ofthe catadioptric group (or in the coatings 41" and 45" only). UV lightscattered by features on the sample surface 89 are imaged by thecatadioptric imaging system of FIG. 1, beginning with the catadioptricgroup, then through the achromatic field lens group, and focusing lensgroup, to the tube elements 71, 73, 75 and 77 of the tube system (absentthe illumination group 61-69).

Ring dark field illumination can be used instead of the directional darkfield illumination of FIG. 5. In that case, a ring illumination source,such as a ring shaped flash lamp, shines a ring or partial ring of lightthrough a matching hole or partially reflective area of the coating inthe hemispherical reflector. This can be done for more than one angle ofincidence of light on the object to be observed.

In yet another alternate embodiment, the objective can be used as a domescatterometer. The reflective components in such a system may be coatedwith long pass, short pass or bandpass, diffuse, fluorescent coatings.The optical components themselves are then imaged onto a set ofdetectors placed around the hemispheric reflector to measure the fullsky scattering pattern from the fluorescent emission of the coatings.Alternatively, a dichroic or partially reflective and partiallytransmissive hemispheric mirror coating would allow direct measurementof scattered light transmitted through the coating.

FIG. 6 shows a wafer inspection apparatus that can use the catadioptricimaging system as a UV objective 86 for the apparatus. The apparatus maybe constructed according to one or more of U.S. Pat. Nos. 4,247,203;4,556,317; 4,618,938; and 4,845,558 of the assignee of the presentinvention. A semiconductor wafer 82 with a plurality of integratedcircuit die 84 at some stage of formation on the wafer 82 is shown lyingin a carrier or stage 80. The stage 80 is capable of providing movementof the wafer 80 with translational X and Y and rotational θ motioncomponents relative to a UV microscope objective 86, such as thecatadioptric imaging system seen in FIG. 1. Light 83 collected from adie 84 or a portion of a die and formed into a magnified image of thatdie or portion by the objective 86 is transferred through a relay lensor lens system 90, such as the tube lens system seen in FIG. 4, into theaperture of a video or CCD array camera 92 sensitive to deep UV light.The output 94 of the camera 92 is fed into a data processor 96, whichcompares pixel data relating to the UV image of the die or die portioneither to data corresponding to other portions of the image or to storeddata from previous images relating to other die or other die portions.The results of this comparison are fed as data 98 to an output device,such as a printer or a CRT display, or to a data storage unit.

One advantage of the broad band UV objective lens of the presentinvention with lateral color correction is its large field size of about0.5 mm diameter, compared to prior narrow band UV lenses that have afield size on the order of 0.1 mm or less. This provides a field with atleast 25 times greater area, allowing for high speed inspection of awafer surface, reticle or similar object. Inspections that previouslytook 20 to 30 minutes to complete can now be done in about one minute.The new lens also has a significantly flattened field, which is a mustfor surface viewing and inspection. Note that no broad band UV objectivepreviously exists. The current Zeiss Ultrafluor 100× objective needstuning a ring and refocus in order to be used at a specific wavelength.

However, the most important advantage is the objective'smulti-wavelength capability. Prior UV objectives are relatively narrowband designs in which good performance is limited to single wavelengthsources, because of significant chromatic aberrations over wavelengthbands as small as 10 nm in the deep UV (e.g., near 248 nm). In manyapplications, multi-wavelength sources, such as Xenon flash lamps andarc lamps, are the preferred light source, due to their low cost andabsence of coherent artifacts. Such sources demand primary and residuallongitudinal and lateral color correction over a broader wavelength bandof at least 20 nm, and preferably over 100-200 nm wide bands. In othercases, multiple light sources at widely different wavelengths may beused in a single system, again demanding broad band color correction inthe UV spectrum.

For a wafer fab facility with both i-line (365 nm) and deep UV 248nm-based steppers, the broad band UV lens of the present inventionenables a reticle inspection system to have selectable i-line or 248 nmwavelength illumination to match the exposure wavelength for which areticle or mask has been constructed. Such wavelength matching isimportant, for example, for inspecting advanced phase shifting masks.Likewise, the broad band UV lens of the present invention allows forconstruction of a system with selectable wavelength for improvedinspection of photoresist on wafers. Photoresist is a material that istransparent to visible light, providing low contrast for inspection atthose wavelengths. However, photoresist becomes opaque at the shorter UVwavelengths with different resists becoming opaque at differentwavelengths. Thus, wafers with i-line photoresist can be inspected withhigh sensitivity at a wavelength of about 313 nm, where it is opaque.Wafers with deep UV (248 nm) photoresist can be inspected at a differentwavelength around 220 nm. The lens system of the present inventionallows the same inspection apparatus to inspect both kinds ofphotoresist.

In a similar fashion, multiple wavelength imaging of UV light can helpin understanding the observed image. For example, different materialsvary in their reflectivities at different UV wavelengths. That is, theyhave what by analogy to color in visible light might be termed "UVcolor". Most metals other than aluminum become opaque, while siliconbecomes more reflective in deep UV light. If combined with a UV camerahaving a UV photodetector imaging array and a combination of wavelengthselective UV transmission filters, the broad band UV objective lens ofthe present invention can be used to provide a "UV color" image of theobject being inspected. This would be useful in defect and featureclassification on a wafer. The UV imaging array can be made, forexample, with Cs₂ Te, Cs₃ Sb, K₂ CsSb or GaAsP detector elements.Silicon-based back-thinned or microlens array CCDs have also beendeveloped for UV imaging.

Likewise, a system can be built that analyzes materials based onfluorescence. Many materials, including most organic materials, such asphotoresists, fluoresce, but they respond to different excitationwavelengths and they emit at different fluorescence wavelengths. Withthe broad band UV imaging lens of the present invention, a fluorescencedetection system can be built with adjustable wavelength from about 0.2to 0.4 μm. By analyzing the fluorescence wavelength, the compositions ofthe materials being observed can be determined. The UV reflectivecomponents of the catadioptric system can be coated with long pass,short pass, or bandpass dichroic coatings to image the fluorescencesignals while rejecting the reflected or scattered excitation light.

The depth of focus of an optical system (proportional to wavelength andinversely proportional to the square of the system's numerical aperture)is intrinsically very short in the ultraviolet spectrum (typically onthe order of 0.1 to 0.5 μm). This can create a problem in imagingpatterned wafers and other similar surfaces with nonplanar profiles.With the broadband UV optics of the present invention, we can usemultiple UV wavelength imaging at different depths and computer softwareintegration of the resulting images to extend the depth of focus toabout 1 μm. For example, we can scan the surface of a wafer or otherobject at three different UV colors with about a 10 to 50 nm wavelengthseparation (e.g., at 0.20, 0.22 and 0.24 μm) using three different focalplanes for the different wavelengths to image different slices of thesurface. A confocal microscope configuration with the UV objective ofthe present invention and with three detectors having correspondingbandpass filters could be used for this purpose. The three images canthen be integrated by a computer to produce a composite with theincreased depth of focus. The small depth of focus of the high N.A. lenssystem can also be used to advantage to produce high resolution imageslices at various depths that can be integrated to form a 3-D image.

The UV objective lens of the present invention is useful in manydifferent microscopy techniques, including the previously mentionedbright field, dark field and fluorescence techniques, as well as phasecontrast, polarization contrast, differential interference contrast andother techniques. For example, the system may be used in a confocalmicroscope geometry using a UV lamp and full field imaging instead of ascanning laser device. Some or all of these techniques can be usedsimultaneously or in sequence within the same objective lens.

We claim:
 1. A broad-band deep-ultraviolet achromatic catadioptricimaging system, comprising:a focusing lens group including a pluralityof lens elements, all formed from a single refractive material withrefractive surfaces thereof disposed at first predetermined positionsalong an optical path of the system and having curvatures and saidpositions selected to focus ultraviolet light at an intermediate imagewithin the system, and simultaneously to also provide in combinationwith the rest of the system, high levels of correction of both imageaberrations and chromatic variation of aberrations over a wavelengthband including at least 0.20-0.29 μm, a field lens group with a netpositive power disposed along said optical path proximate to saidintermediate image, the field lens group including a plurality of lenselements formed from at least two different refractive materials withdifferent dispersions, with refractive surfaces of the lens elements ofthe field lens group disposed at second predetermined positions andhaving curvatures selected to provide correction of chromaticaberrations including at least secondary lateral color of the systemover said wavelength band, a catadioptric group including a firstoptical element having at least a concave reflective surface with acentral optical aperture therein disposed along said optical pathproximate to said intermediate image so that ultraviolet light from theintermediate image can pass therethrough, the catadioptric group alsoincluding a second optical element which is a lens with a reflectivemirror coating on a rear surface of said lens except for a central areaon said rear surface where said mirror coating is absent, the opticalelements of said catadioptric group being arranged such that ultravioletlight from the intermediate image transmitted through said centraloptical aperture of said first optical element of said catadioptricgroup passes through the lens portion of said second optical element ofsaid catadioptric group, reflects from said reflective mirror coating onsaid lens rear surface, passes back through said lens portion towardssaid concave reflective surface of said first optical element, isreflected thereby and passes a third time through said lens portion ofsaid second optical element and through said central area of said lensrear surface to form a final image beyond said catadioptric group. 2.The imaging system of claim 1 wherein said wavelength band includes0.20-0.40 μm.
 3. The imaging system of claim 1 wherein said wavelengthband includes 0.193 μm.
 4. The imaging system of claim 1 wherein saidsingle refractive material type of said focusing lens group is fusedsilica.
 5. The imaging system of claim 1 wherein said field lens groupis formed from lens elements made of fused silica and a fluoride glass.6. The imaging system of claim 1 wherein said first optical element ofsaid catadioptric group comprises a concave mirror with a central holetherein forming said central optical aperture.
 7. The imaging system ofclaim 1 wherein said first optical element of said catadioptric groupcomprises a meniscus lens with a concave reflective surface coatingthereon.
 8. The imaging system of claim 7 wherein said central opticalaperture is formed by a central hole in said meniscus lens.
 9. Theimaging system of claim 7 wherein said central optical aperture isformed by a central area on said meniscus lens where said concavereflective surface coating is absent.
 10. The imaging system of claim 1wherein said catadioptric group is characterized by reflective surfacecurvatures selected to provide at least a 0.8 numerical aperture and a0.5 mm field of view for said final image of the imaging system.
 11. Abroad-band deep-ultraviolet achromatic catadioptric imaging system,comprising:a first negative lens, a second positive biconvex lensclosely spaced to the first lens to form a substantially zero-powercorrector group for chromatic variations of image aberrations, a thirdnegative meniscus lens spaced from the second lens, a fourth negativemeniscus lens with concave surfaces of the third and fourth lensesfacing each other, a fifth positive biconvex lens, a sixth positivemeniscus lens, a seventh meniscus lens of near zero-power with concavesurfaces of the sixth and seventh lenses facing each other, the thirdthrough seventh lenses being closely spaced together to form a focusinglens group with minimal image aberrations over a specified wavelengthband that includes at least 0.22-0.29 μm ultraviolet wavelengths, saidfocusing lens group providing an intermediate image, eighth, ninth andtenth lenses forming a field lens group positioned proximate to saidintermediate image, said field lens group being substantially achromaticover said specified wavelength band, said field lens group including atleast one positive convex lens of a different refractive material typethan all other lenses in the system and at least one negative meniscuslens, the field lens group having a net positive power, an eleventhnegative meniscus lens with a convex surface facing said first lenshaving a reflective coating thereon and with a first central opticalaperture therein proximate to said intermediate image, and a twelfthnear zero-power, substantially flat lens with a reflective coating on asurface facing away from the first lens with a second central opticalaperture therein, said twelfth lens spaced apart from the eleventh lens,the eleventh and twelfth lenses with their respective reflectivecoatings forming a catadioptric group providing a light focusing relayfor the intermediate image to provide a final image proximate to thesecond optical aperture for which chromatic aberrations aresubstantially corrected in said final image over said specifiedwavelength band.
 12. The imaging system of claim 11 wherein the thirdlens is spaced at least 30 mm from the second lens, and said twelfthlens is spaced at least 30 mm from the eleventh lens.
 13. The imagingsystem of claim 11 wherein the focusing lens group, field lens group andcatadioptric group have refractive and reflective surfaces that arecharacterized by the following dimensional values:

    ______________________________________                                                 Radius of                                                            Surface #                                                                              Curvature (mm)                                                                            Spacing (mm) Material                                    ______________________________________                                        1        1105.7      4.000        fused silica                                2        53.051      2.500        air                                         3        284.061     5.000        fused silica                                4        -57.181     60.000       air                                         5        39.782      15.000       fused silica                                6        13.379      7.977        air                                         7        -12.955     5.169        fused silica                                8        -17.192     1.000        air                                         9        42.964      8.000        fused silica                                10       -55.269     1.000        air                                         11       18.436      8.000        fused silica                                12       91.574      6.253        air                                         13       -20.802     4.000        fused silica                                14       -21.768     17.120       air                                         15       7.217       5.494        fused silica                                16       2.259       3.000        CaF.sub.2 glass                             17       -11.760     1.500        fused silica                                18       373.721     39.730       air                                         19       flat        7.000        fused silica                                20       flat        -7.000       reflector/                                                                    fused silica                                21       flat        -36.000      air                                         22       50.470      -9.500       fused silica                                23       64.290      9.500        reflector/                                                                    fused silica                                24       50.470      36.000       air                                         25       flat        7.000        fused silica                                26       flat        1.500        air                                         ______________________________________                                    


14. The imaging system of claim 11 wherein the focusing lens group,field lens group and catadioptric group have refractive and reflectivesurfaces that are characterized by the following dimensional values:

    ______________________________________                                                 Radius of                                                            Surface #                                                                              Curvature (mm)                                                                            Spacing (mm) Material                                    ______________________________________                                        1        -67.007     4.000        fused silica                                2        50.308      2.000        air                                         3        120.297     6.000        fused silica                                4        -37.494     30.636       air                                         5        24.138      10.000       fused silica                                6        13.441      9.532        air                                         7        -13.518     7.546        fused silica                                8        -17.997     1.000        air                                         9        34.465      6.000        fused silica                                10       -517.022    1.000        air                                         11       18.268      10.000       fused silica                                12       965.352     4.181        air                                         13       -30.177     9.746        fused silica                                14       -28.138     7.892        air                                         15       -19.346     2.500        fused silica                                16       -36.530     1.000        air                                         17       6.687       5.026        fused silica                                18       2.044       0.017        air                                         19       2.044       2.000        CaF.sub.2 glass                             20       -90.635     36.108       air                                         21       -908.968    7.000        fused silica                                22       -1000.0     -7.000       reflector/                                                                    fused silica                                23       -908.968    -36.000      air                                         24       48.244      -9.500       fused silica                                25       63.204      9.500        reflector/                                                                    fused silica                                26       48.244      36.000       air                                         27       -908.968    7.000        fused silica                                28       -1000.0     1.500        air                                         ______________________________________                                    


15. A broad-band deep-ultraviolet achromatic catadioptric imagingsystem, comprising:a focusing lens group including a plurality of lenselements, with refractive surfaces thereof disposed at firstpredetermined positions along an optical path of the system and havingcurvatures and said positions selected to focus ultraviolet light at anintermediate image within the system, and simultaneously to also providein combination with the rest of the system, high levels of correction ofboth image aberrations and chromatic variation of aberrations over awavelength band including at least 0.22-0.29 μm, a field lens group witha net positive power disposed along said optical path proximate to saidintermediate image, the field lens group including a plurality of lenselements formed from at least two different refractive materials withdifferent dispersions, with refractive surfaces of the lens elements ofthe field lens group disposed at second predetermined positions andhaving curvatures selected to provide substantial correction ofchromatic aberrations including at least secondary longitudinal colorand primary and secondary lateral color of the system over saidwavelength band, a catadioptric relay group, including a combination ofat least two reflective surfaces and at least one refractive surfacedisposed at third predetermined positions and having curvatures selectedto form a real final image of said intermediate image such that, incombination with said focusing lens group, primary longitudinal color ofthe system is substantially corrected over said wavelength band.
 16. Theimaging system of claim 15 wherein all of said lens elements of saidfocusing lens group are formed from a single refractive material type.17. The imaging system of claim 16 wherein said single refractivematerial type of said focusing lens group is silica.
 18. The imagingsystem of claim 15 wherein said catadioptric relay group includes afirst reflective optical element with a concave reflective surface, saidconcave reflective surface having a central optical aperture thereindisposed along said optical path proximate to said intermediate image sothat ultraviolet light from the intermediate image can passtherethrough, said catadioptric relay group also including a secondoptical element which is a lens with a reflective mirror coating on arear surface of said lens except for a central area on said rear surfacewhere said mirror coating is absent, said concave reflective surface ofsaid first reflective optical element and said reflective mirror coatingof said second optical element forming said at least two reflectivesurfaces of said catadioptric relay group, a front surface of said lensof said second optical element forming said at least one refractivesurface of said catadioptric relay group,said optical elements of saidcatadioptric relay group being arranged such that ultraviolet light fromthe intermediate image transmitted through said central optical aperturein said concave reflective surface of said first reflector opticalelement passes through the lens portion of said second optical element,reflects from said reflective mirror coating on said lens rear surface,passes back through said lens portion towards said first reflectiveoptical element, is reflected by said concave reflective surface of saidfirst reflective optical element, and passes a third time through saidlens portion of said second optical element and through said centralarea of said lens rear surface where said reflective mirror coating isabsent to form a final image beyond said catadioptric relay group. 19.The imaging system of claim 18 wherein said first reflective opticalelement of said catadioptric relay group comprises a concave mirror witha central hole therein forming said central optical aperture.
 20. Theimaging system of claim 15 wherein said at least two differentrefractive materials with different dispersions forming said lenselements of said field lens group includes both silica and a fluorideglass.